ULP Advisor > Rule 7.1 Use local instead of global variables where possible

ULP Advisor - Rule Table ULP 1.1 Ensure LPM usage ULP 2.1 Leverage timer module for delay loops ULP 3.1 Use ISRs instead of flag polling ULP 4.1 Terminate unused GPIOs ULP 5.1 Avoid processing-intensive operations: modulo, divide. ULP 5.2 Avoid processing-intensive operations: floating point ULP 5.3 Avoid processing-intensive operations: (s)printf() ULP 6.1 Avoid multiplication on devices without hardware multiplier ULP 6.2 Use MATHLIB for complex math operations ULP 6.3 Use Low Energy Accelerator (LEA) software library ULP 7.1 Use local instead of global variables where possible ULP 8.1 Use 'static' & 'const' modifiers for local variables ULP 9.1 Use pass by reference for large variables ULP 10.1 Minimize function calls from within ISRs ULP 11.1 Use lower bits for loop program control flow ULP 11.2 Use lower bits for port bit-banging ULP 12.1 Use DMA for large memcpy() calls ULP 12.1b Use DMA for potentially large memcpy() calls ULP 12.2 Use DMA for repetitive transfer ULP 13.1 Count down in loops ULP 14.1 Use unsigned variables for indexing ULP 15.1 Use bit-masks instead of bit-fields Let us know what you think! Feedback, suggestions & comments are welcome @ ULPAdvisorFeedback@list.ti.com

What it means

C compilers for micro-controllers in general and for MSP430 MCUs specifically allocate the variables declared in the global scope into the RAM location of the memory structure. On the other hand, local variables are first allocated into CPU registers (until the function runs out of the CPU registers for its local variables). Accesses to CPU registers are the most efficient and quickest, when compared to accesses to memory locations in RAM or even worse, in FLASH. Therefore, any global variables that are only accessed within one function can be safely relocated into the function local scope to optimize the memory access speed and power consumption.

Risks, Severity

Using global variables that can be otherwise be declared locally could decrease the memory access speed efficiency and hence increase power consumption.

Why it is happening

ULP Advisor detects a static global variable that is only accessed within one function of the project.

Remedy

Place the variable in question in the function locally instead of declaring it globally.

Code Example

See the rest of the code examples for all MSP430 devices here!

More Resources

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